"There exist various coordinate reference systems in which a geographic
location may be
described mathematically by coordinates. In each system, the position gets its own coordinate
values. These values differ in a numerical sense, but represent the same geographic position.

When collecting data stored in different coordinate reference systems, each
definition has to be
known together with its geometric relationship to a standard system. Only then, it is possible to
transfer all data into a standard coordinate reference system."

Stefan A. Voser

Fig. 1: From the earth surface to the plane of a
map projection

The main coordinate reference systems for describing a
geographic position mathematically are
geodetic reference systems and map projections (Fig. 1).

Geodetic reference systems are used for describing
the figure of the Earth and positions on it:
ellipsoids (and the sphere) are used for describing the horizontal position, whereas geoids
are the
gravity related model for referencing the elevation. Geodetic reference systems have a datum,
describing the position and orientation of the model in relation to the Earth and its surface.